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A 76-year-old woman presented to the emergency department after seeing blood in her urine. She explained that this had been going on since the previous day, when she also began to experience episodes of pain on the right side of her abdomen, between her ribs and hip. She had no underlying medical conditions.

She was not feverish, and had no discomfort associated with urination. A renal punch proved to be negative. Clinicians performed a bedside ultrasound, which revealed moderate hydronephrosis affecting her right kidney.

A CT intravenous pyelogram showed a lobulated mass measuring 1.6 cm in the right renal pelvis. It was mildly hyperdense (38 HU) on pre-contrast imaging, and heterogeneously enhanced on nephrographic phase (78 HU) but less than that of normal parenchyma (121 HU), with an attenuation ratio of 0.64.

Cytology testing of the urine showed no malignant cells. Suspecting a urinary collecting system tumor, clinicians performed a ureteroscopy, which revealed a renal pelvic mass. A biopsy of the lesion revealed carcinoma cells in nests and tubules, polygonal to round, with pleomorphic and hypochromatic nuclei.

Other cytology findings included:

• Expression of PAX8

• Positive for napsin A, 34betaE12, and cytokeratin 7 (CK7)

• Negative for tumor protein p63 and GATA3 (markers for transitional cell carcinoma [TCC])

• Positive focally for alpha-methylacyl-CoA racemase (AMACR)

The patient subsequently underwent whole-body staging CT, which revealed that the mass had continued to grow, and hydronephrosis had progressed. Imaging showed no thoracoabdominal lymphadenopathy, and bone scan showed no sign of metastasis to bone.

When clinicians performed a radical nephrectomy, the tumor had remained confined to the kidney (pT1a). It had a growth pattern resembling TCC, predominantly tubular, and tubulopapillary architecture. There was no evidence that it was attached to renal parenchyma. Nevertheless, its renal origin was confirmed by cytologic evidence of PAX8/napsin A positivity and the absence of p63/GATA3.

Discussion

Clinicians presenting this very rare of a patient with renal cell carcinoma (RCC) confined to the renal pelvis noted that differentiating an RCC from a TCC is crucial in order to determine the type of surgery required -- nephrectomy or nephroureterectomy -- and the need for more extensive lymphadenectomy. "Intensive surveillance for metachronous tumors in the remnant urinary tract is often needed for patients with TCC," the authors noted.

Because about 90% of cancers located in the renal pelvis are TCCs, that is the presumptive etiology of any "renal mass with its epicenter in the pelvicaliceal system," they wrote. While histopathological evidence indicates that up to 14% of RCCs involve the urinary collecting system, they said, it is extremely rare to encounter one located exclusively in the collecting system. They cited a similarly unusual case of a patient with hereditary leiomyomatosis and RCC syndrome.

Some theories for how this occurs include the idea that a tumor developing marginally has easier access to the adjacent hollow structure; that initial implantation of carcinoma cells may occur via the urothelial mucosa, followed by intraluminal expansive growth; and that tumor cells may spread via intraluminal transit down the urinary tract. The authors suggested that the latter explanation likely accounts for their patient's RCC, given that the urothelial cells around this tumor were normal.

Diagnosis may be aided by CT imaging features and enhancement pattern, they observed; however, ureteroscopy and histopathology results "should be considered as the criterion standards and routine preoperative assessment."

The group elaborated on the value of dynamic CT to determine the nature of a renal pelvic lesion: "A high-density mass on pre-contrast scan may indicate calculus, blood clot, or neoplasm," they wrote, with the latter confirmed by evidence of enhancement of >15 HU.

According to a of centrally infiltrating renal masses on CT, six CT features were found to be most diagnostically specific for identifying intrarenal TCCs: locating the center of the tumor to any part of the collecting system was the most valuable, with a mean specificity of 88.5% and a mean sensitivity of 79.3%. Its clinical utility was further supported by good inter-observer agreement. A preserved renal outline was the most reproducible sign, with an inter-observer agreement of 0.69, a specificity of 81.1%, and a sensitivity of 78.0%. The other four signs -- a focal filling defect in the collecting system, absence of cystic or necrotic change, homogeneous tumor enhancement, and tumor seen to extend down to the ureteropelvic junction -- had specificities in the range of 79-87%, with moderate inter-observer agreement.

The case authors noted that a tumor's location and potential renal parenchyma involvement are determined via a three-dimensional CT intravenous pyelogram. In general, RCCs show greater enhancement than TCCs due to their hypervascular nature, they added.

They cited a study of during unenhanced corticomedullary, nephrographic, and excretory phases in 20 patients with RCC and 12 with TCC, compared with the attenuation of the normal renal cortex. This study found significant differences in the attenuation ratios between RCC and TCC in the corticomedullary (0.77 vs 0.50) and nephrographic (0.72 vs 0.45) phases using three small regions of interest, but no significant differences between RCCs and TCCs in any phase when measuring the complete tumor lesion.

These findings suggested that RCC is likely to demonstrate greater isodensity than a normal kidney, the authors noted; this phenomenon was evident in their patient, "where the tumor-to-kidney attenuation ratio was 0.64 on nephrographic phase. In cases where features of RCC such as necrosis, cystic degeneration, hemorrhage, and calcification are absent, such an enhancement pattern may suggest an alternative diagnosis."

Identifying the appropriate surgical management -- nephrectomy or nephroureterectomy -- depends on differentiation between RCC and TCC, the authors said, since TCC requires a wider lymphadenectomy followed by intensive monitoring of the urinary tract for subsequent independent primary malignancies.

Although reports are inconsistent regarding outcomes with RCCs involving the urinary collecting system, there are linking this manifestation of RCC to reductions in both , especially in patients with localized disease, the authors explained.

For this reason, ureteroscopy and histopathology should be viewed as key elements of routine preoperative imaging assessment. This is particularly important in cases of RCC located only in the collecting system, which can be difficult to diagnose, the authors concluded.

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